1. Field of the Invention
The present invention is directed to an improved production process of manufacturing holograms and more particularly to an improved dehydrating step for manufacturing dichromated gelatin holograms in a safe and expedited manner.
2. Description of the Related Art
The advantages of using holograms for various displays is well known. Commercially, holographic optical elements are being used for numerous purposes such as HUD, Head-Up Displays, head-down displays, laser eye protection visors, helmet-mounted displays, simulator domes and solar concentrators. There is a desire to provide a production capacity to meet the demands for these holographic optical elements.
It has been known to use hardened gelatin films as the medium for the recordation of information by holography. Usually the hardened gelatin film is sensitized to exhibit a light response by contacting it with an appropriate sensitizer solution. The sensitized gelatin film is then exposed in the usual holographic manner and then the gelatin plate is subsequently desensitized and developed to be stabilized for a period of time commensurate with the expected life of the holographic optical element.
An example of a pre-exposure hardened gelatin film can be found in U.S. Pat. No. 3,617,274. Additionally, a discussion of the various types of dichromatic gelatin and procedures for developing the same can be found in "Topics in Applied Physics, Holographic Recording Materials", Volume 20, 1977, edited by H. M. Smith.
The related art has proposed hardening of the gelatin before processing or during the liquid processing development of the holographic images. Most of these processes use a thermal or actinic agent to harden the gelatin via cross-linking with dichromate ions. There have been other attempts to use gelatin hardeners such as aldehydes or commercial fixers with a hardener (a photographic fixer itself has no fixing affect on non-silver halide films). The effect of cross-linking the gelatin is to reduce the efficiency of the hologram. Attempts to restore efficiency by processing at hotter temperatures presents the problem of making the gelatin softer. Consequently, no net gain was obtained.
It is known that the reflection wavelength exhibited by dichromated gelatin reflective holographic optical elements display a logarithmic function of time and temperature. The rate of change with time varies by a factor of about 2.2 for every 10.degree. C. change in temperature. A standard method of achieving thermal stability in the production of a HUD hologram is to bake it for extended times at relatively high temperatures. Approximately a one-month period of baking or stabilizing at 100.degree. C. will insure that the rate of change in a wavelength with time will be so slight that virtually no measurable further change will occur at the operating temperatures during the usable life of a holographic optical element in a HUD.
As noted by T. A. Schenkoff in "Applied Optics", Volume 7, Page 2101 (1968), an optically-induced phase shift of an exposed and developed hardened dichromatic gelatin plate could be increased by changing the development process of the hologram. This article suggested developing the hologram in mildly agitated water at 35.degree. C. for approximately 30 seconds, dipping the gelatin plate in isopropanol for 30 seconds and then air drying with a stream of dry air udder pressure. The gelatin was saturated with water and fully swollen at the time it was immersed in the isopropyl alcohol and the alcohol was used to replace the water in the gelatin. The isopropyl alcohol dried the gelatin layer extremely rapidly because it did not dissolve the gelatin and it did not wet it. It was further observed that the developed hologram could be reimmersed in water and any large index of refraction change would disappear and the hologram efficiency would be returned to a small value. It was noted that the process was reversible if the wet layer was again dried in isopropyl alcohol. Thus, it has been known that a photochemical cross-linking that is produced during the exposure of the dichromated gelatin will remain intact during subsequent steps and that it controls whatever process occurs during the dehydration step. With a hardened gelatin plate, the gelatin molecules are tied together into a continuous three-dimensional network. During swelling and shrinking, this network is basically not changed and so the basic photochemical information recorded in the gelatin layer is not destroyed during development in water and remains in the volume independent of the thickness of the gelatin layer. It has been recognized, however, that the change in the thickness will affect the properties of the hologram and that generally, the thickness of the fully developed emulsion should approximately be equal to that of the gelatin layer during the exposure for minimum aberrations in the diffracted image. Usually, dichromated gelatin utilizes a development procedure which includes a first water-washing step to soften the gelatin for the following drying step. The water-washing step also removes the unreacted dichromate to thereby prevent crystallization and the introduction of scattering centers upon drying.
The washed gelatin plate is usually immersed wet into a water/isopropyl alcohol mixture and a plurality of baths can be utilized with the last bath being as free of water as possible. There have been proposals to control the dissipation of the alcohol from the developed gelatin plate to maintain the hologram efficiency and to discourage any crystallization of the gelatin plate. Brandes et al., "Applied Optics", Volume 8, Page 2346 (1969), disclosed that light scattering and the sensitivity increased with increasing wash-water temperature and decreased when the sensitized gelatin plates had been hardened more extensively. He proposed developing harened gelatin plates in a water-wash at 25.degree. to 40.degree. C. followed by dehydration with isoprop-yl alcohol at 70.degree. C.
Meyerhofer, at Page 90 of "Topics in Applied Physics, Holographic Recording Materials" (1977), proposed preparing a gelatin layer by dip coating with 12 to 18 percent by weight gelatin, from J. T. Baker Chemical Company, USP. Grade, 125 Bloom, suspension in water which was initially mixed at 20.degree. C. with a final stirring at 70.degree. C. and filtering with a heated filter. The mixture was then cooled to 40.degree. C. and (NH.sub.4).sub.2 Cr.sub.2 O.sub.7, ammonium dichromate, was added to give a ratio of 0.5 percent to the weight of the gelatin. The glass plate substrate was inserted and withdrawn vertically at a rate of 1 to 5 centimeters per minute. The coated plates were then air dried for 1 hour in a vertical position and then subsequently hardened by baking at 150.degree. C. for 2 hours. The thickness of the gelatin coating on the plate was in the range of 4 um. The films were dried in the dark and stored at 20.degree. C. or lower for more than 12 hours before exposure. After exposure, the plate was washed for approximately 10 minutes in running water at 20.degree. C. The plate was then soaked for 2 minutes in a mixture of 50 percent isopropyl alcohol and 50 percent distilled water with mild agitation. The plate was then removed and the procedure repeated for 2 minutes in a bath of 90 percent isopropyl alcohol and 10 percent water. Finally, the hologram plate was inserted vertically into a fresh isopropyl alcohol bath with agitation for 10 to 20 minutes as the final bath. The plate was pulled out of the bath at a rate of about 1 centimeter per minute while a flow of hot air was directed against the gelatin.
At ordinary room atmosphere conditions, it was found that dichromated gelatin holograms were relatively stable. The use of a combination of water and alcohol bath was believed to be necessary to prevent an excessive rate of dehydration that could create a high-stress condition and cause a milky-white appearance in the gelatin plate which produced heavy light scattering. In mass production, the use of a highly flammable isopropyl alcohol can be dangerous, particularly at elevated temperatures.
The demand for high quality display holographic combiners such as required in the production of head-up displays remain. There is still a demand for improved production processes that can maintain the optical quality while increasing the production yield in a safe environment.